Method of making a valve stem

ABSTRACT

A valve stem has a thickness of hardfaced material on its outer diameter in the area contacting the stem packing where the seal is formed which prevents corrosion, erosion and/or scratching of the valve stem, thereby preventing formation of leak paths.

This application is a Divisional of Ser. No. 09/032,871 filed Mar. 2,1998 U.S. Pat. No. 6,068,018.

TECHNICAL FIELD

The present invention relates generally to valves of the type used inthe petrochemical, refining, and other industries, and more particularlyto an improved valve stem which is resistant to chemical and mechanicaldeterioration, and to a method of manufacturing the valve stem.

BACKGROUND AND SUMMARY OF THE INVENTION

Perhaps the most common valves used in the petrochemical, refining, andother industries are gate valves. Major components of gate valvesinclude a body, a valve stem, a sealing member, and seat rings which areengaged by the sealing member to close the valve. Because the valve stemmust extend from inside the valve body to outside the body, a stempacking must be installed to effectively seal between the body and thevalve stem, thereby containing the product inside the piping system ofwhich the valve is a part. To achieve the necessary dynamic seal to bothcontain the product within the piping system and allow the valve stem toactuate up and down, the area of the valve stem which makes contact withthe stem packing must be smooth, round and have no taper on the outsidesurface of the valve stem.

When the valve is new, the foregoing conditions are met and productinside the piping system is prevented from leaking to the outside. Asthe valve ages, chemical corrosion from the product, galvanic corrosionfrom the packing, and erosion and corrosion from atmospheric conditionsall combine to reduce the smoothness of the valve stem that is necessaryto maintain the seal. Also, roughness of the valve stem can occur as thevalve stem is pulled through the stem packing during normal cycling dueto abrasive materials accumulating between the valve stem and the seal.Any of these factors, and particularly combinations thereof, can resultin leak paths being created and leakage to occur. This is unsatisfactoryfor numerous reasons, the most important of which involves increasingenforcement of the federal Clean Air Act which makes prevention ofleakage or “fugitive emissions” essential.

Previous attempts at solving the foregoing problems have included theapplication of the material sold under the trademark “HASTALLOY”®. Suchmaterial is partially effective in slowing valve stem deterioration dueto chemical attack but is ineffective as to mechanical degradation ofthe valve stem surface.

While the prior art discloses a variety of valves, no prior disclosureshows a device where leakage is prevented by way of an overlay on thevalve stem in the area of the stem packing. The improved valve stem ofthe present invention departs from the design of prior valve stems, andin doing so, provides a valve which prevents leakage and fugitiveemissions as the valve ages.

SUMMARY OF THE INVENTION

The present invention provides an improved valve in which the valve stemhas an overlay of hardfaced material that reduces the deterioration ofthe stem packing sealing surface. The hardfaced overlay is constructedof materials which resist corrosion, erosion, and scratching, thuseliminating the cause of most defects in the sealing surface of thevalve stem. This in turn allows maintaining the original packing areafinish, thereby preventing the creation of leak paths through the stempacking and fugitive emissions from the valve.

In accordance with more specific aspects of the invention, the portionof the valve stem that engages the sealing material has a nominaloutside diameter which is first reduced, either during manufacture ormodification of the valve stem. Hardfaced material is applied to thereduced diameter portion of the valve stem until the diameter thereof isincreased beyond the nominal size. The diameter of the hardfacedmaterial is next reduced to the nominal size, thus providing a valvestem that is smooth, hard, and resistant to chemical and mechanicaldeterioration.

BRIEF DESCRIPTION OF DRAWINGS

A more complete understanding of the invention may be had by referenceto the following Detailed Description when taken in conjunction with theaccompanying Drawings, in which:

FIG. 1 is a sectional view of a gate valve having a valve stemconstructed in accordance with the invention;

FIG. 2 is a side view of the improved valve stem of the invention;

FIG. 3 is a cross-sectional view of the valve stem of FIG. 2;

FIG. 4 is an illustration of a first step in the method of theinvention;

FIG. 5 is an illustration of a later step in the method of theinvention; and

FIG. 6 is an illustration of a still later step in the method of theinvention.

DETAILED DESCRIPTION

Referring now to the Drawings, and in particular to FIG. 1 thereof,there is shown an improved valve 10 having a body 12 constructed toprovide a fluid path 14 and a body cavity 16. The fluid path 14 isgenerally located in the lower end 18 of body 12, while the cavity 16extends from the upper end 20 of body 12 to the location of the fluidpath 14. Disposed within the fluid path 14 are seat ring recesses 22 andseat rings 24 mounted in the seat ring recesses 22.

A valve member 28 is slidably positioned in the body 12 for movementthrough the cavity 16 such that sealing portions 30 of the valve member28 may engage the seat rings 24. The valve member 28 is operablyconnected to a valve stem 32 which extends through a stem packing 34located between the cavity 16 and a bonnet 35 mounted on the body 12.

The valve member 28 is actuated between the open and closed positions byan actuating member 36. The improved valve 10 is in the open positionwhen the valve member does not engage the seat rings 24, and is in theclosed position when the sealing portions 30 of the valve member 28engage the seat rings 24 thereby completely restricting the flow ofproduct P within the fluid path 14. The valve member 28 can be anymember used to restrict or close flow through the improved valve 10, butin the preferred embodiment the valve member 28 is a wedge.

As shown in FIG. 1, the actuating member 36 may comprise a handwheel 38secured to a threaded portion 41 of the valve stem 32. The threadedportion 41 is threadingly engaged with a nut 40 which is secured againstrotation. As the handwheel 38 is turned clockwise, the nut 40 and thethreaded portion 41 cause the valve member 28 to move downward until thevalve 10 is in the closed condition. If the handwheel 38 is rotatedcounterclockwise, the nut 40 and the threaded portion 41 effect openingthe valve 10. Although not shown, other actuating members well known inthe art are also within the scope of this invention.

As the valve member 28 is moved during the opening and closing of thevalve 10, the valve stem 32 slidingly engages the stem packing 34. Thestem packing 34 is installed such that when the valve stem 32 slidinglyengages the stem packing 34, a dynamic seal is effectively formedbetween the body 12, the bonnet 35, and the valve stem 32. The sealcontains the product P found within the piping system of which the valve10 is a part.

As the valve stem 32 moves up and down through stem packing 34, problemsarise in the prior art because as the valve stem 32 ages, chemicalcorrosion from the product, galvanic corrosion from the stem packing 34,and both corrosion and erosion from the atmosphere cause a normal valvestem to lose the smoothness necessary to prevent damage to the stempacking 34, which causes a leak path to form. Also, abrasive materialsaccumulating between the packing and the stem can cause scratching ofthe surface of the valve stem, also causing leak paths. In severe cases,chemical and mechanical deterioration occur in combination.

As shown in FIG. 2, this problem is remedied by a thickness of hardfacedmaterial 42 on the section 43 of the outside diameter of the valve stem32 which slidably engages the stem packing 34. The hardfaced material isa material resistant to corrosion, erosion, and scratching and includesmaterials with Brinell hardness numbers ranging from 220 to 380, such asStoody® 6 manufactured by Stoody Company, 5557 Nashville Road, BowlingGreen, Ken. 42101-7546, and Stellite® 6 and Stellite® 21 manufactured byDeloro Stellite, Inc., 471 Dundas St. E. Belleville, Ontario, Canada K8N1G2. Materials with Brinell hardness numbers approximately 220 and belowlack the required hardness to resist corrosion, erosion or scratching,while materials with Brinell hardness approximately 380 and above createproblems in the machining process because they are very hard to machineand cause maintenance problems for the machining equipment. In thepreferred embodiment, the hardfaced material 42 comprises eitherStellite® 6 or Stellite® 21 with a Brinell number range of 320-380.

For an existing valve stem, the hardfaced material 42 is placed on thevalve stem 32 by the following process. First, the outside diameter ofan existing valve stem is machined down approximately 0.120 inches usinga conventional machine tool. Next, the outside diameter of the valvestem 32 is built back up using a gas metal arc welding (GMAW) process toproduce a section of hardfaced material 42 having an approximate totalthickness of 0.200 to 0.250 inches. The now oversized valve stem 32 ismachined back to its original diameter and may be polished required byparticular applications of the invention. In the preferred embodimentusing Stellite® 21, the finished valve stem 32 has a section ofhardfaced material 42 having a thickness of approximately 0.090 inches.Other embodiments will call for a larger or smaller thickness rangingfrom 0.010 to 0.125 inches of hardfaced material 42. The exact thicknessrequired for a particular application will depend upon the requirementsof particular applications of the invention and/or by the corroded andscratched condition of the valve stem sought to be fitted with thethickness of hardfaced material.

The hardfaced material may also be applied using the plasma transfer arc(PTA) process. In the PTA process one or more powders, which may includemetals and/or metallic salts and/or non-metallic materials, are directedinto a plasma which melts the powdered material prior to transfer to thesurface being treated. The use of the PTA process to apply the hardfacedmaterial is beneficial in that the composition of the hardfaced materialcan be matched to the requirements of particular applications of theinvention.

A new valve stem with a thickness of hardfaced material 42 and having aknown diameter can be produced by the following method. First, a basematerial, such as stainless steel in the preferred embodiment, is forgedor molded into a base stem having a diameter smaller than the nominaldiameter, at least in the region that engages the packing. The moldingprocess can be done by any method known in the art for forging, castingand/or molding metal. The diameter of the base stem will usually rangebetween 0.010 to 0.125 inches smaller than the nominal diameter. In thepreferred embodiment using Stellite® 6 or Stellite® 21, the diameter ofthe base stem will be 0.090 inches smaller than the known diameter. Thediameter of the base stem can be varied according to the requirements ofparticular applications of the invention.

Once the base stem has been molded, hardfaced material 42, such as thatpreviously disclosed, is welded onto the base stem until the diameter isgreater than the nominal diameter. This can be done by the use of a gasmetal arc welding (GMAW) process, providing an approximate totalthickness of 0.125 to 0.250 inches of hardfaced material 42. The nowoversized valve stem is machined back to its nominal diameter andpolished. As in the preferred embodiment using Stellite® 6 or Stellite®21, the finished valve stem has a thickness of hardfaced material ofapproximately 0.090 inches. Other embodiments will call for a larger orsmaller thickness ranging from about 0.010 inches to about 0.125 inchesof hardfaced material. The exact thickness required for a particularapplication will be dictated by the requirements of particularapplications of the invention.

The hardfaced material may also be applied using the plasma transfer arc(PTA) process. In the PTA process one or more powders, which may includemetals and/or metallic salts and/or non-metallic materials, are directedinto a plasma which melts the powdered material prior to transfer to thesurface being treated. The use of the PTA process to apply the hardfacedmaterial is beneficial in that the composition of the hardfaced materialcan be matched to the requirements of particular applications of theinvention.

Referring to FIGS. 4, 5, and 6, the method of the present invention isillustrated. A valve stem 44 is provided with a portion 46 having areduced diameter relative to the nominal diameter 48 of the valve stem.The reduced diameter portion comprises the area of the valve stem 44which engages the seal of the valve. The reduced diameter portion 46 maybe provided either in the original manufacture of the valve stem 44, orby grinding or otherwise machining a preexisting valve stem to providethe reduced diameter portion 46.

The reduced diameter portion 46 is filled with a layer of hardfacedmaterial 50 which is applied to and secured to the valve stem 44 bywelding. The welding of the hardfaced material 50 continues until theouter surface 52 thereof extends beyond the nominal diameter 48 of thevalve stem 44. Thereafter, the hardfaced material is ground and polishedto provide an exterior surface 54 thereof which is coincident with thenominal diameter 48 of the valve stem 44.

Although preferred embodiments of the invention are illustrated in theaccompanying Drawings and described in the foregoing DetailedDescription, it will be understood that the invention is not limited tothe embodiments disclosed, but is capable of numerous rearrangements,modifications, and substitutions of parts and elements without departingfrom the spirit of the invention.

I claim:
 1. A method for producing a hardfaced valve stem from anexisting valve stem having a first diameter comprising the steps of: a.machining a section of said existing stem to a second diameter, saidsecond diameter being less than said first diameter; b. weldingsufficient hardfaced material to said section to increase said seconddiameter to a third diameter greater than said first diameter; and c.machining said section to reduce said third diameter to approximatelyequal said first diameter.
 2. The method according to claim 1 furthercomprising the step of polishing said section.
 3. The method accordingto claim 2 wherein said hardfaced material is selected from a group ofmaterials consisting essentially of one or more materials having aBrinell hardness number between 220 and
 380. 4. The method according toclaim 1 wherein said hardfaced material is selected from a groupconsisting essentially of Stellite® 6, Stellite® 21, or combinationsthereof, and having a thickness between 0.010 inches and 0.125 inches.5. The method according to claim 4 wherein said predetermined thicknessis about 0.090 inches.
 6. A method for producing a new valve stem of apredetermined diameter comprising a base material with a thickness ofhardfaced material secured thereto comprising the steps of: a. moldingsaid base material into a base stem having a diameter less than saidpredetermined diameter; b. welding sufficient said hardfaced material tosaid base stem to increase the diameter of said base stem to greaterthan said predetermined diameter; and c. machining said base stem to asufficient amount to reduce its diameter to said predetermined diameter.7. The method according to claim 6 further comprising the step ofpolishing said hardfaced material.
 8. The method according to claim 6wherein said hardfaced material is selected from a group consistingessentially of one or more materials having a Brinell hardness numberbetween 220 and
 380. 9. The method according to claim 8 wherein saidhardfaced material has a thickness between about 0.010 inches and about0.125 inches, and is constructed from Stellite® 6, Stellite® 21, orcombinations thereof.
 10. The method according to claim 9 wherein saidpredetermined thickness is about 0.090 inches.